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. In December 2002 and January 2003 satellite observationsof chlorophyll showed a strong coastal signal along the west African coastbetween 10 and 22∘ N. In addition,a wavelike pattern with a wavelength of about 750 kmwas observed from 20 December 2002 and was detectablefor 1 month in the open sea, south-west of the Cape Verde Peninsula.Such a pattern suggests the existence of a locally generatedRossby wave which slowly propagated westward during this period. This hypothesiswas confirmed by analysing sea surface height provided by satellite altimeter duringthis period. To decipher the mechanisms at play, a numerical studybased on a reduced-gravity shallow-water model has first been conducted. A wind burst,broadly extending over the region where the offshore oceanic signal is observed,is applied for 5 d. A Kelvin wave quickly develops along the northern edgeof the cape, then propagates and leaves the area in a few days.Simultaneously, a Rossby wave whose characteristics seem similarto the observed pattern forms and slowly propagates westward.The existence of the peninsula limits the extent of the waveto the north. The spatial extent of the wind burstdetermines the extent of the response and correspondingly the timescaleof the phenomenon (about 100 d in the present case). When the wind bursthas a large zonal and small meridional extent, the behaviourof a wave to the north of the peninsula differsfrom that to the south.These results are corroborated and completed by an analytical study of a linearreduced-gravity model using a non-Cartesian coordinate system. This system isintroduced to evaluate the potential impact of the coastline shape.The analytical computations confirm that a period of around 100 dcan be associated with the observed wave considering the value of the wavelength;they also show that the role of the coastline remains moderate at such timescales.By contrast, when the period becomes shorter (smaller than 20–30 d),the behaviour of the waves is modified because of the shape of the coast.South of the peninsula, a narrow band of sea isolated from the restof the ocean by two critical lines appears. Its meridional extent is about 100 kmand Rossby waves could propagate there towards the coast.

Generation of Rossby waves off the Cape Verde Peninsula: the role of the coastline